Just call them up and ask them to add it. Of course, if you used a specific awd, and this awd is not associated with your wizard number, your rate may change (for better or for worse). If you want to use a specific awd, just log in as usual using your wizard number, on the appropriate screen change the awd to the one you prefer and tick the box that says something like 'check the lowest rate', and then just continue to the end. Then cancel the first reservation.

PS The problem may be that avis.com requires both your wizard number and your last name